1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a facsimile, a printer and an all-in-one machine having a paper dust removing device which removes adhered materials such as paper dust (sheet dust) on sheets.
2. Description of the Related Art
Generally, an image forming apparatus such as a copying machine, a facsimile, a printer and an all-in-one machine uses various sheets such as plain paper, recycled paper, OHP resin sheets and coated paper, and is demanded to form high quality images on these various sheets.
Here, when plain paper is fed, paper dust (sheet dust) is produced from the surface and end portion of plain paper, and therefore paper dust removing devices of various systems are conventionally arranged in image forming apparatuses to remove paper dust on sheets. However, to remove paper dust on the sheets, when, for example, an adhesive member is used for the paper dust removing device, there is a problem that running cost increases for an exchange of the adhesive member due to a decrease of an adhesive force. Further, a paper dust removing device which uses a positive/negative charging brush, positive/negative charging roller, ultrasonic wave, air sucking, and air blowing causes problems of increased power consumption and increased noise.
Hence, a paper dust removing device is proposed which solves the above various problems as much as possible by varying paper dust removing performance of the paper dust removing device according to sheets and enabling adjustment of paper dust removing performance.
The paper dust removing device in Japanese Patent Laid-Open No. 2000-335762 has the paper dust removing member which faces a sheet conveying path and the paper dust removing power source which applies a bias voltage to the paper dust removing member. By applying bias voltages of a charge polarity and opposite-polarity of paper dust to the paper dust removing member, it is possible to electrostatically attract paper dust of sheets to the paper dust removing member and remove paper dust. By increasing the absolute value of the bias voltage to be applied to this paper dust removing member, it is possible to increase an electrostatic attracting force and improve paper dust removing performance.
Further, a control device can adjust paper dust removing performance of the paper dust removing device based on the sheet state determined by a determination device. For example, when the determination device determines that sheets are coarse paper, the paper dust removing performance is increased by increasing the absolute value of the bias voltage. By contrast with this, when sheets are coated paper or OHP resin sheets of comparatively less paper dust, the paper dust removing performance may be decreased by decreasing the absolute value of the bias voltage, or set to off by turning off the bias voltage.
By this means, it is possible to prevent sheets from being unnecessarily charged, prevent power consumption from increasing and suppress image noise produced when sheets are unnecessarily charged.
Thus, by adjusting paper dust removing performance according to sheets, it is possible to reduce the frequency of exchanges, power consumption or noise of a paper dust removing member of the adhesive member.
However, the technique discusses in Japanese Patent Laid-Open No. 2000-335762 causes the following problem. For sheets (coarse paper in Japanese Patent Laid-Open No. 2000-335762) such as recycled paper which are likely to produce paper dust, the absolute value of the bias voltage is increased and paper is fed by maximizing the paper dust removing performance, and then, for the sheets (for example, coated paper or OHP resin sheets) which are not likely to produce paper dust, the bias voltage is turned off. However, when paper is fed by turning off the bias voltage in this way and minimizing paper dust removing performance, there are problems that images on coated paper and OHP resin sheets cause image deterioration (image defect) such as white dots and splattering due to paper dust.
This is because paper dust adheres to a sheet conveying path after recycled paper which produces many paper dust is fed. Then, paper dust adhered to the sheet conveying path is transferred onto coated paper or OHP resin sheets. This is because, with coated paper or OHP resin sheets, the bias voltage is turned off and paper dust removing performance is minimized, and therefore paper dust transferred onto coated paper or OHP resin sheets from the sheet conveying path cannot be removed, thereby causing white dots and image splattering.
That is, when, for example, an operator who is going to form images start printing (image formation), if low quality sheets which produce a great mount of paper dust are used upon printing (upon last before) before (immediately before) current printing, paper dust produced upon the last printing is left on the conveying path. Then, a problem is caused that paper dust adheres to the sheets which are conveyed upon subsequent printing, thereby causing image deterioration such as white dots and splattering.
It is therefore an object of the present invention to provide an image forming apparatus which realizes an adequate operation of a paper dust removing device by adjusting paper dust removing performance taking the influence of paper dust produced upon previous (last) image formation with respect to sheets used upon current image formation, and which does not deteriorate images due to paper dust.